st/mesa: generate GL_OUT_OF_MEMORY if we can't create the index buffer

Before, if we failed to allocate the index buffer we'd silently
return from st_draw_vbo() without drawing anything.  We should
raise GL_OUT_OF_MEMORY to give some indication that something went
wrong.

Note: This is a candidate for the stable branches.

Reviewed-by: Marek Olšák <maraeo@gmail.com>

src/mesa/state_tracker/st_draw.c

@@ -230,7 +230,7 @@ st_draw_vbo(struct gl_context *ctx,
                                    nr_prims);
 
       if (!setup_index_buffer(st, ib, &ibuffer)) {
-         /* out of memory */
+         _mesa_error(ctx, GL_OUT_OF_MEMORY, "glBegin/DrawElements/DrawArray");
          return;
       }